The molecular structures and conformations of various tumor inhibitors and related compounds isolated from plant sources will be determined by single-crystal X-ray diffraction methods. The determinations will be attempted for compounds where the complexity of the chemical problems involved in normal chemico-physical study does not allow an unambiguous assignment of structure and, in particular, for compounds where the amount of material isolated from the plant is too small to allow a chemical approach. We will also be carrying out structure determinations on simpler materials where the rapidity of the direct X-ray method makes this a competitive technique with standard chemical attack. Compounds of known structure may be examined where unique information on molecular conformation, not obtainable by other methods, may be required. Examples of tumor inhibitors whose structures have been determined in the course of this project with the help of X-ray studies include: maytansine, triptolide, tripdiolide, jatrophone, tetrandrine, eriolanin and eriolangin, and steganacin and steganangin. BIBLIOGRAPHIC REFERENCES: Structure of Gnidicoumarin, a Novel Pentacyclic Dicoumarin from Gnidia latifolia, S.M. Kupchan, R.L. Baxter, J.G. Sweeny, T. Murae, M.S. Shen, and R.F. Bryan, Chemical Communications, 94 (1975); Podolide, a New Antileukemic Norditerpene Dilactone from Podocarpus gracilior, S.M. Kupchan, R.L. Baxter, M.F. Ziegler, P.M. Smith, and R.F. Bryan, Experientia, 31, 137 (1975).